Prostanoids have been implicated in the pathogenesis of bronchoconstriction and airway hyperreactivity (AHR) because they potently contract airway smooth muscle (ASM), and their levels, are elevated in asthmatic lung. Thromboxane, PGD2 and PGF2alpha contract ASM via activation of thromboxane prostanoid (TP) receptors. Even PGE2, which is generally considered a bronchodilator, has the capacity to transduce a constrictor signal via one of the receptor subtypes (EP1) for which it is the cognate ligand. However, despite their potent effects on ASM, it has been difficult to unambiguously assign pathophysiological relevance to these receptors due to pleiotropic responses resulting from the simultaneous production of constrictor and relaxant prostanoids by the same general enzymatic pathway, receptor activation by multiple ligands, and the existence of multiple receptor isoforms with different signal transduction properties. We believe these limitations can be overcome by using transgenic and gene-targeted mice to selectively modulate signal transduction by specific prostanoid receptors. In this proposal, we will test the overall hypothesis that bronchoconstriction and AHR occur when the activation of bronchoconstrictor prostanoid receptors is favored over those that promote bronchodilation. In Specific Aim 1, we will use mice that overexpress the TP receptor in combination with TP receptor deficient gene-targeted mice to determine whether TP receptors promote bronchoconstriction and AHR. The signaling pathways, which mediate TP receptor-dependent contraction will be dissected in vitro using primary ASM cells, isolated from the genetically engineered mice, and their physiologic significance determined in tracheal rings and intact mice. In Specific Aim 2, we will determine whether the EPt receptor is a mediator of bronchoconstriction and AHR. The transgenic models developed for these experiments will enable us to discriminate between the effects of EP1 receptor activation and those of the other EP receptor subtypes. Since it widely recognized that the primary physiologic actions of one G-protein-coupled receptor are frequently modulated by another, experiments in Specific Aim 3 will test the hypothesis that receptor cross talk by TP and EP1 receptors is a mechanism of (-adrenergic receptor dysfunction in ASM. This hypothesis is supported by preliminary data that show TP and EP1 receptor agonists attenuate isoproterenol-mediated relaxation of tracheal rings. Each of these aims will merge biochemical, pharmacologic and physiologic studies from cells, tissues, and whole animals so that in vitro signaling events can be directly correlated to in vivo physiological function. Completion of these aims may identify novel signaling events that have therapeutic relevance to asthma and other obstructive lung diseases.